The protective immune response to L. monocytogenes is used to investigate cell-mediated immunity to intracellular bacteria. A necessary virulence determinant is secretion of listeriolysin O (LLO). Listeria-specific CD8+ T cells are cytotoxic in vitro, these effectors lyse L. monocytogenes infected targets. In BALB/c mice (H2d), CD8+ CTLs have been isolated which are Kd restricted, and a nanomer peptide corresponding to aa91-99 of LLO shown to be presented by the MHC Class Ia molecule. LLO-specific CD8+ T-cells have also been described which lyse L. monocytogenes infected targets in an MHC Class IB restricted manner; the antigen presenting molecule is encoded within the T-region of a less polymorphic family of MHC Class I molecules. Studies in Aim 1 will determine if LLO is an immuno-dominant antigen for MHC Class Ib restricted antilisterial CTLs. Studies in Aim 2 will determine if MHC Class Ib restricted anti-listerial cells are protective in vivo. Studies in Aim 3 will identify the LLO-derived MHC Class Ib-restricted CTL epitope. Results from these studies will provide an understanding of the role of MHC Class Ib restricted effector cell in the protective immune response to an intracellular pathogen.